Yordan Pavlov created ARROW-11897:
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Summary: [Rust][Parquet] Use iterators to increase performance of
creating Arrow arrays
Key: ARROW-11897
URL: https://issues.apache.org/jira/browse/ARROW-11897
Project: Apache Arrow
Issue Type: Improvement
Components: Rust
Reporter: Yordan Pavlov
The overall goal is to create an efficient pipeline from Parquet page data into
Arrow arrays, with as little intermediate conversion and memory allocation as
possible. It is assumed, that for best performance, we favor doing fewer but
larger copy operations (rather than more but smaller).
Such a pipeline would need to be flexible in order to enable high performance
implementations in several different cases:
(1) In some cases, such as plain-encoded number array, it might even be
possible to copy / create the array from a single contiguous section from a
page buffer.
(2) In other cases, such as plain-encoded string array, since values are
encoded in non-contiguous slices (where value bytes are separated by length
bytes) in a page buffer contains multiple values, individual values will have
to be copied separately and it's not obvious how this can be avoided.
(3) Finally, in the case of bit-packing encoding and smaller numeric values,
page buffer data has to be decoded / expanded before it is ready to copy into
an arrow arrow, so a `Vec` will have to be returned instead of a slice
pointing to a page buffer.
A decoder output abstraction that enables all of the above cases and minimizes
intermediate memory allocation is `Iterator)>`.
Then in case (1) above, where a numeric array could be created from a single
contiguous byte slice, such an iterator could return a single item such as
`(1024, &[u8])`.
In case (2) above, where each string value is encoded as an individual byte
slice, but it is still possible to copy directly from a page buffer, a decoder
iterator could return a sequence of items such as `(1, &[u8])`.
And finally in case (3) above, where bit-packed values have to be
unpacked/expanded, and it's NOT possible to copy value bytes directly from a
page buffer, a decoder iterator could return items representing chunks of
values such as `(32, Vec)` where bit-packed values have been unpacked and
the chunk size is configured for best performance.
Another benefit of an `Iterator`-based abstraction is that it would prepare the
parquet crate for migration to `async` `Stream`s (my understanding is that a
`Stream` is effectively an async `Iterator`).
Then a higher level iterator could combine a value iterator and a (def) level
iterator to produce a sequence of `ValueSequence(count, AsRef<[u8]>)` and
`NullSequence(count)` items from which an arrow array can be created
efficiently.
In future, a higher level iterator (for the keys) could be combined with a
dictionary value iterator to create a dictionary array.
Finally, Arrow arrays would be created from a (generic) higher-level iterator,
using a layer of array converters that know what the value bytes and nulls mean
for each type of array.
[~nevime] , [~Dandandan] , [~jorgecarleitao] let me know what you think
Next steps:
* split work into smaller tasks that could be done over time
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